Process and an apparatus for producing calcium carbonate via an enzymatic pathway

ABSTRACT

The invention relates to a method for improving the production yield of precipitated calcium carbonate (PCC). The improvement resides in the use of an enzymatic bioreactor that supplies HCO 3  into the reaction enabling to produce PCC without bubbling gaseous carbon dioxide in the aqueous solution. The present invention has an industrial applicability, namely in the pulp and paper industry.

FIELD OF THE INVENTION

The present invention generally relates to the field of calciumcarbonate production obtained by precipitation. More particularly, itconcerns a process for preparing precipitated calcium carbonate(hereinafter referred to as PCC) involving the use of an enzyme thatdirectly supplies a source of bicarbonate ions which are required by thePCC production process. The present invention is particularly useful inthe pulp and paper industry.

BACKGROUND OF THE INVENTION

Production of CaCO₃

Calcium carbonate is used in different fields, most importantly in thepulp and paper industry, where it is largely used for coating the paperafter it has been pressed, thereby increasing the printing quality ofthe paper, as well as its color, smoothness, and strength.

The traditional process of forming solid CaCO₃ consists in bubblingcarbon dioxide in milk of lime (Ca(OH)₂). This process allows asufficient amount of CO₂ to solubilize and produce solid calciumcarbonate according to the following equation:Ca(OH)₂+CO₂<-------->CaCO₃+H₂O  [1]

This reaction is a rather slow reaction and one drawback thusencountered with the same is its low production yield as compared to theneed of CaCO₃ in the pulp and paper industry or in other fields.

There is thus presently a need for a process for the production of CaCO₃that provides a better production yield.

Conversion of CO₂ into Hydrogen Ions and Bicarbonate Ions

EP0991462; AU7753398; WO9855210; CA2291785 in the name of the Applicantdisclose the use of the enzyme carbonic anhydrase to catalyse thehydration of CO₂ into hydrogen ions and bicarbonate ions. The reactionthat allows for the production of bicarbonate in the presence of theenzyme is represented by equation [2]:

The hydration kinetics of CO₂, with or without enzyme, has been theobject of several scientific works. The reaction rate constant ofnon-catalysed (without enzyme) reactions is in the order of 0,035s⁻¹(25° C., pH=7)¹. At an elevated pH (pH>10), the hydroxyl ions (OH⁻)contribute to the catalysis, and consequently, the reaction rateconstant can attain 8.5×10³ s⁻¹ at 25° C.². The catalysed reaction (withenzyme), can also be associated with an elevated reaction rate constantin the order of 1,4×10⁶ (25° C., pH=8.8)³. The ratio of the two reactionrate constants allows one to predict the CO₂hydration kinetics, whichcan be in the order of 50 million times more elevated when an enzyme ispresent and depending on the experimental conditions.1 (Carbonic Anhydrase: Zinc and the Mechanisms of Catalysis. Biology andChemistry of the Carbonic Anhydrolases; Annals of the New-York Academyof Sciences, 429 (1984) pp.26 to 48, (Coleman J. E.); “The CarbonicDioxide Hydration activity of carbonic anhydrase I. Stop-Flow Kineticstudies on the Native Human Isoenzymes B and C”, Journal of BiologicalChemistry, 246(8) (1971), pp. 2561 to 2573 (Khalifah, R. G.); Biochimie(2000) DeBoeck Université (Garett et al.) 2 ((Carbonic Anhydrase: Zincand the Mechanisms of Catalysis. Biology and Chemistry of the CarbonicAnhydrolases; Annals of the New-York Academy of Sciences, 429 (1984)pp.26 to 48, (Coleman J. E.); Carbonic Anhydrase Kinetics and MolecularFunction: The carbonic Anhydrase, Plenum Press, pp. 49 to 69 (1991),Khalifah et al.) 3 (“The Carbonic Dioxide Hydration activity of carbonicanhydrase I. Stop-Flow Kinetic studies on the Native Human Isoenzymes Band C”, Journal of Biological Chemistry, 246(8) (1971), pp. 2561 to 2573(Khalifah, R. G. )

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process and anapparatus for the production of CaCO₃ that satisfies the above-mentionedneed for a process that provides an increased production yield ofprecipitated calcium carbonate (PCC). In accordance with the presentinvention, that object is achieved with a process for producing CaCO₃,charaterized in that it comprises the steps of:

-   -   a) catalysing the hydration of CO₂contained in a CO₂-containing        gas by means of an enzyme capable of catalysing the hydration of        dissolved CO₂into hydrogen ions and bicarbonate ions, thereby        producing a solution containing bicarbonate ions and hydrogen        ions;    -   b) reacting the bicarbonate ions contained in the solution        obtained in step a) with calcium ions, thereby producing a        solution containing CaCO₃; and    -   c) precipitating the CaCO₃ contained in the solution obtained in        step b).

The enzyme used in the process is preferably carbonic anhydrase or ananalogue thereof.

In accordance with a preferred aspect, step b) comprises the step ofmixing the solution obtained in step a) with Ca(OH)₂, thereby providingthe calcium ions. In that case, the CaCO₃ is thus produced from milk oflime (Ca(OH)₂) and CO₂as in the prior art, although it shows an improvedyield, as explained hereinafter. As mentioned above, the traditionalprocess of forming CaCO₃ consists in bubbling carbon dioxide in milk oflime (Ca(OH)₂). This process produces solid calcium carbonate accordingto the following equation:Ca(OH)₂+CO₂<-------->CaCO₃+H₂O  [1]

This non catalyzed solubilization of gaseous CO₂in an aqueous mediumimplies the following series of natural reversible reactions. Theequilibrium constants of these natural reactions are well known in theprior art.CO₂+H₂O<-->H₂CO₃ K≅600 à25° C. (K≡[CO₂]/[H₂CO₃])  [3]H₂CO₃<->H⁺+HCO₃ ⁻ K=4,47×10⁻⁷ à25° C. (K=[HCO₃ ⁻][H⁺]/[H₂CO₃])  [4]HCO₃ ⁻<-->H⁺+CO₃ ²⁻ K=4,70×10⁻¹¹à25° C. (K=[CO₃ ²⁻][H⁺]/[HCO3⁻])  [5]

From a thermodynamic point of view, the large equilibrium constantassociated to equation [3] translates into a weak inherent tendency toproduce the acid (K_(eq)=[CO₂]/[H₂CO₃]≅600) (Chemistry of the Elements(₂nd edition, Butterworth Heinemann, p. 310 (1997)).

Equation [3], representing the formation of carbonic acid (H₂CO₃), is inreality the limiting step of the process for producing bicarbonate.

Thus, the use of an enzyme, preferably carbonic anhydrase, which isspecific to CO₂molecules, avoids the step of carbonic acid formation. Inother words, the enzyme catalyses the hydration of CO₂by eliminating thelimiting step of the process. As mentioned above, the reaction thatallows for the production of bicarbonate in the presence of the enzymeis represented by equation [2]:

Reaction [2] allows for the production of an aqueous solution having ahigh HCO₃ ⁻ ion content that is used in the process of producing PPC.This concept is represented in the following equation:HCO₃ ⁻H⁺+Ca(OH)₂<-->CaCO₃+2H₂O  [6]

From equations [1] and [6], one can see that the reaction for producingPCC performs better in the presence of an enzymatic bioreactor suppliedwith gaseous CO₂which in turn directly supplies HCO₃ ⁻ to the processfor preparing precipitated calcium carbonate. The improvement residesnot only in the increased hydration rate of CO₂, but also in theincreased amounts of CaCO₃ formed in a given reaction time. Thevariation of Gibbs free energy (ΔG) for each of the reactions willindeed allow one to determine which of the two chemical reactions isfavourable from a thermodynamic point of view.

Reaction [6], having a ΔG in the order of −119 kJ/mol, is morefavourable from a thermodynamic point of view than reaction [1] whichhas a ΔG value in the order of −74 kJ/mol. The respective equilibriumconstant of these two chemical reactions is calculated using equation[7]:−ΔG=RT In (K _(eq))  [7]

The equilibrium constant of a chemical reaction not containing enzyme isabout 9,36×10¹² at 298K, while a catalysed reaction making use of anenzyme is about 7,24×10²⁰. While the equilibrium constant is higher fora reaction containing an enzyme, the chemical equilibrium is morefavourable towards the product (CaCO₃) then to the reagents, andconsequently, a better yield is reached.

In accordance with a preferred aspect of the invention, the step a) ofCO₂hydration is performed in a bioreactor comprising a reaction chamberfilled with the enzyme and step b) is performed in at least one separatereaction tank, the process further comprising a step of directing a flowof the solution from the bioreactor into the reaction tank.

Also preferably, step c) of precipitating is performed in the reactiontank. In the present context, precipitating the CaCO₃ means theformation of separable solid CaCO₃ from the solution or crystallizationof the CaCO₃.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a first installation suitablefor producing precipitated calcium carbonate according to the process ofthe present invention.

FIG. 2 is a schematic representation of a second installation suitablefor producing precipitated calcium carbonate according to the process ofthe present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention consists in replacing the step of bubbling gaseous CO₂inthe milk of lime by a direct input of HCO₃ ⁻ in an aqueous solution soas to accelerate the production rate of CaCO₃.

Referring to FIGS. 1 or 2, a CO₂conversion bioreactor (1) having areaction chamber (2) filled with carbonic anhydrase is used. Preferably,the reaction chamber (2) is filled with packing (3) on which carbonicanhydrase is immobilized.

The most common form which the CO₂conversion bioreactor (1) can take isthe one containing a packed column where gaseous CO₂or other effluentscontaining CO₂penetrate towards the bottom of column (5) at a highconcentration (>10 000 ppm) and exit by the outlet (6) at aconcentration in the order of 500 ppm. As for example, a bioreactor asthe one described in WO9855210; CA2291785 is suitable for performing theprocess. The dimensions of the column depend on both the ratio of gasvs. liquid flow rate and the difference of concentrations between theinlet (5) and the outlet (6). The liquid phase, containing water and abuffer solution so as to allow the pH of the enzymatic medium to bemaintained, flows into the bioreactor by an upper inlet (7) and crossesthe gas flow while flowing towards the bottom of the reaction chamberwhere the outlet (8) is located. During the crossing, water, CO₂, andthe enzyme react according to equation [2]. The concentration of gaseousCO₂then decreases since water fills with HCO₃ ⁻ and H⁺ ions inequivalent proportions upon transformation of the CO₂into HCO₃ ⁻ and H⁺ions. The gas concentration at the outlet (6) is an indicator of theamount of HCO₃ ⁻ dissolved. However, it is preferable to measure theamount of dissolved ions in the solvent (8) found in the pipes.

The solution containing the ions is then directed towards one or morereaction tanks (9 or 9′) that contain the milk of lime. It is worthmentioning that the milk of lime is preferably kept at an optimalconcentration so as to produce homogenous PCC crystals. Turning now toFIG. 2, the reaction tanks (9 and 9′) are used in parallel and can benumerous. The temperature at which milk of lime is kept can be adjustedin the conditioning tank (10) by means of heating/cooling system (11).This system allows the solution to be either heated or cooled dependingon the HCO₃ ⁻ solution properties. Afterwards, an exact amount of milkof lime is added into the reaction tanks (9 and 9′) by means of avolumetric pump (4). Indeed, when the reagents are placed together,CaCO₃ crystals are formed within the first moments of contact. Thecontent of the reaction tank (9 and 9′) is preferably stirred by meansof conventional mixer (12 or 12′) to prevent the CaCO₃ crystals fromsettling.

Finally, the content of each of the reaction tanks (9 and 9′) isalternatively transferred one after another into a buffer tank (13)where the liquid and the calcium carbonate solution await to enter thefiltration system (15) where they are to be separated from one another.A mixing system (14) can be used to avoid the formation of deposits(sedimentation) along the bottom of the buffer tank (13). The use of abuffer tank (13) enables the reaction tanks (9 and 9′) to remainavailable to receive solutions so as to keep the process continuous.

It is worth mentioning that it is also possible to directly add theenzyme in free form into the reaction chamber (9 or 9′). However, indoing so, it can compromise the precipitate purity as well as consuminglarge amounts of enzyme. Indeed, immobilization of anhydrase on thepacking (3) allows the enzyme to be reused many times over until itsperformance is no longer optimal.

The process and the apparatus according to the invention areadvantageously applicable in the industry of pulp and paper forimproving the production yield of CaCO₃ used for coating the paper.

It is however worth mentioning that the invention could also beadvantageous in other fields requiring the use of CaCO₃.

Although preferred embodiments of the present invention have beendescribed in detail herein and illustrated in the accompanying drawings,it is to be understood that the invention is not limited to theseprecise embodiments and that various changes and modifications may beeffected therein without departing from the scope or spirit of thepresent invention.

1. A process for producing CaCO₃, charaterized in that it comprises the steps of: a) catalysing the hydration of CO₂contained in a CO₂-containing gas by means of an enzyme capable of catalysing the hydration of dissolved CO₂into hydrogen ions and bicarbonate ions, thereby producing a solution containing bicarbonate ions and hydrogen ions; b) reacting the bicarbonate ions contained in the solution obtained in step a) with calcium ions, thereby producing a solution containing CaCO₃; and c) precipitating the CaCO₃ contained in the solution obtained in step b).
 2. A process according to claim 1, characterized in that step b) comprises the step of: mixing the solution obtained in step a) with Ca(OH)₂, thereby providing said calcium ions.
 3. A process according to claim 1 or 2, characterized in that said enzyme is carbonic anhydrase or an analogue thereof.
 4. A process according to any one of claims 1 to 3, characterized in that step a) of hydration of CO₂is performed in a bioreactor comprising a reaction chamber filled with said enzyme and the step b) is performed in at least one separate reaction tank, the process further comprising a step of directing a flow of said solution from said bioreactor into said reaction tank.
 5. A process according to claim 4, characterized in that the reaction chamber is filled with packing on which the enzyme is immobilized.
 6. A process according to any one of claims 1 to 5, characterized in that step b) is performed under stirring to prevent the calcium carbonate from settling.
 7. A process according to any one of claims 1 to 6, characterized in that it comprises an additional step of: d) separating the precipitate of CaCO₃ of step c) from the solution.
 8. A process according to claim 6, characterized in that step d) of separating consists of filtering.
 9. An apparatus for producing CaCO₃ according to the process defined in claim 1, characterized in that it comprises: catalyzing means for catalysing the hydration of the CO₂into bicarbonate ions and hydrogen ions; reacting means for reacting the bicarbonate ions obtained in the catalyzing means with calcium ions to produce CaCO₃; and precipitating means for precipitating the CaCO₃ obtained in the reacting means.
 10. An apparatus according to claim 9, characterized in that the means for catalyzing the hydration of the CO₂comprises a bioreactor comprising: a gas inlet for receiving gaseous CO₂; a liquid inlet for receiving an aqueous liquid; a reaction chamber in fluid communication with the gas inlet and the liquid inlet, the reaction chamber containing therein enzymes capable of catalysing the hydration of dissolved CO₂into bicarbonate ions and hydrogen ions; and a liquid outlet in fluid communication with the reaction chamber for discharging a solution of bicarbonate ions and hydrogen ions.
 11. An apparatus as claimed in claim 10, characterized in that the enzyme is carbonic anhydrase.
 12. An apparatus as claimed in claim 10 or 11, characterized in that the means for reacting the bicarbonate ions with calcium ions and precipitating CaCO₃ is at least one reaction tank having an inlet in fluid communication with the liquid outlet of the bioreactor and an outlet to discharge a solution containing CaCO₃.
 13. An apparatus as claimed in claim 12, characterized in that it comprises a buffer tank having an inlet in fluid communication with the outlet of the at least one reaction tank for receiving and reserving the solution obtained in said at least one reaction tank for a further treatment.
 14. An apparatus according to claim 13, characterized in that it comprises a filter in fluid communication with said buffer tank to separate the CaCO₃ from the solution. 